1. Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to an improved front assembly for color tubes having shadow masks of the tension foil type in association with a substantially flat faceplate. The invention is useful in color tubes of various types, including those used in home entertainment television receivers, and in medium-resolution and high-resolution tubes intended for color monitors.
The use of the foil-type flat tension mask and flat faceplate provides many benefits in comparison to the conventional domed shadow mask and correlatively curved faceplate. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in picture areas of high brightness where the intensity of the electron beam bombardment is greatest. Color impurities result as the mask moves closer to the faceplate, and as the beam-passing apertures move out of registration with their associated phosphor elements on the faceplate. When heated, the tension mask distorts in a manner quite different from that of the conventional mask. If the entire mask is heated uniformly, there is no doming and no distortion until tension is completely lost; just before that point, wrinkling may occur in the corners. If only portions of the mask are heated, those portions expand, and the unheated portions contract, resulting in displacements within the plane of the mask; i.e., the mask remains flat.
The tension foil shadow mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the faceplate. The front assembly comprises the faceplate with its screen consisting of deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about 0.001 inch thick, or less. The mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q-distance." As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of the three beams generated by the electron gun located in the neck of the tube lands only on its assigned phosphor deposits.
The requirements for a support means for a foil shadow mask are stringent. As has been noted, the foil shadow mask is normally mounted under high tension; e.g., 30 lb/inch. The support means must be of high strength so the mask is held immovable; an inward movement of the mask of as little as 0.0002 inch can cause the loss of guard band. Also, it is desirable that the shadow mask support means be of such configuration and material composition as to be compatible with the means to which it is attached. As an example, if the support means is attached to glass, such as the glass of the inner surface of the faceplate, the support means must have a coefficient of thermal contraction compatible with that of the glass, and by its composition, be bondable to glass. Also, the support means must be of such composition and structure that the mask can be secured to it by production-worthy techniques such as electrical resistance welding or laser welding. Further, it is essential that the support means provide a suitable surface for mounting and securing the mask. The material of which the support structure is composed must be adaptable to machining or to other forms of shaping so the structure can be contoured into near-perfect flatness. Otherwise, voids will exist between the metal of the mask and the support structure, preventing positive, uniform contact of the mask to the support structure necessary for proper mask securement.
In the manufacture of a front assembly for a flat tension mask color cathode ray tube, a rail or other member for supporting the shadow mask in tension may be secured to the inner surface of the glass faceplate by a devitrifiable solder glass, sometimes herein termed "frit." The frit is applied between the mask support structure and the faceplate inner surface. The faceplate assembly is elevated in temperature in an oven called a "lehr" which raises the temperature of the assembly to or above the temperature at which the frit devitrifies (crystallizes). At that elevated temperature, the mask support structure becomes rigidly affixed to the faceplate inner surface. As the assembly is cooled, any differential between the net CTC (coefficient of thermal contraction) of the mask support structure and the faceplate glass will create strains in the mask support structure and the glass at their interface. As glass is strong in compression but weaker in tension, there is always the concern that any thermal coefficient mismatch between the mask support structure and the glass will create spalling (tearing away of glass) in the interface area, initiation and propagation of cracks, separation of the mask support structure from the faceplate inner surface, and other such thermal-strain-related defects.
These strains are not so apt to cause problems on a thermal downcycle such as is encountered after devitrification of the frit, as described, since during cool-down, the exterior of the faceplate is cooler than the interior, causing the outer surface of the glass to contract more rapidly than the inner surface. The result of this thermal imbalance is to cause the external surface of the glass to go into tension, and the internal surface of the glass (to which the mask support structure is attached) to go into compression.
Glass in compression is relatively strong and the aforedescribed thermal strain-related defects are not common. However, when the bulb is evacuated, it is again heated to an elevated temperature. Unlike the earlier-described thermal up-cycle wherein the frit was in a liquid state, in this stage of tube fabrication, the mask support structure is rigidly locked to the faceplate inner surface by the devitrified frit. As the bulb exterior is heated, the outside surface of the bulb is placed in compression and the inside surfaces, including the faceplate inner surface to which the mask support structure is secured, are placed in tension. These tensile strains induced in the inner surface of the faceplate will add to any pre-existing tensile strains attributable to a mismatch between the CTC of the mask support structure and the CTC of the faceplate glass.
Such accumulated tensile strain is further augmented by the atmospheric loading on the bulb exterior, particularly the flat faceplate. The effect of these tensile-strain-producing stresses combines to impose limitations on the thermal gradient that the bulb can withstand during tube evacuation and thus on the unit through-put in the evacuation stage.
A cathode ray tube bulb will have in a particular area a strain limit beyond which it is apt to fail. The most critical area on the inner surface of the faceplate has been found to be adjacent the end of the mask support structures wherein there exists bulb shape irregularities, frit-to-glass interfaces, mask support structure terminations, etc. It is important, therefore, that the strain limit in that critical area not be exceeded
A mask support structure which is currently in commercial use by the assignee of the present invention is described in U.S. Pat. No. 4,891,545, of common ownership herewith. The mask support structure described therein comprises a hollow metal trough, the interior of which is filled with conventional color CRT frit, such as Owens Illinois T540 or Corning 7580 series. Frit is used because of its known compatibility with CRT vacuum environments and its neutrality in thermal coefficient with respect to glass. Conventional color CRT frit has a CTC of about 98.times.10.sup.-7 in./in /degree C. The composition of the metal trough is selected to have a CTC most closely approximating that of glass. The preferred material is Alloy No. 27 manufactured by Carpenter Technology, Inc. of Reading, Pa. Alloy No. 27 has a CTC of approximately 108.times.10.sup.-7 in./in./degree C. The mask support structure, representing a combination of frit having a CTC slightly lower than conventional color CRT glass (about 100.times.10.sup.-7 in./in./degree C.) combined with a Carpenter Alloy No. 27 trough, produces a mask support structure having a net CTC which closely approximates the CTC of the faceplate glass. Such a "filled trough" mask support structure can be said to be essentially thermally matched or neutral with respect to the faceplate glass.
It should be noted that conventional color CRT frit has a CTC which is slightly less than the CTC of typical color CRT faceplate glass in order that when used in its customary application to seal a CRT glass funnel (having a CTC of approx. 99.5.times.10.sup.-7 in /in./degree C.) and a faceplate (having a CTC typically about of 100.times.10.sup.-7 in./in/degree C.), the frit will, upon devitrification and cool-down, be placed in compression and thus be stronger than if in tension.
Measurements have been taken of the deflection of a faceplate at various points along a system of mask support structures of the before-described "filled trough" type. (Deflection of a faceplate is indicative of residual strain.) Results of such tests reveal that the stresses imposed on the faceplate by such "filled trough" systems are approximately neutral (about 1200 psi or less), but slightly negative--i.e., in a direction indicating that the frit at the interface between the mask support structure and the glass is in compression (producing a strong frit bond, as desired) and the strain imposed on the glass at the interface is slightly tensile. The sought-after neutrality is essentially achieved by such an arrangement. In summary, following conventional wisdom, the existing commercial flat faceplate color CRT has a mask support structure which is essentially neutral with respect to the faceplate in terms of the CTC's of the mask support structure and the faceplate.
However, the aforedescribed "filled trough" mask support system has a number of shortcomings. First, it has been found to be a thermally "weak" bulb in the sense that it must be heated and cooled at slow rates to keep the tensile strains induced on the inner surface of the faceplate, particularly in the critical areas near the ends of the mask support structure, below the maximum strain tolerance of the bulb. At more acceptable through-put rates, it has been found that cracks develop across the corners of the faceplate near the ends of the mask support structures.
Second, due to the extreme CTC mismatch between the metal trough and the faceplate inner surface, spalling occurs in the sub-structure glass surface--that is, in the area of interface between the mask support structure and the faceplate inner surface--when the front assembly is subjected to thermal shocks. Bond failures, even strip-off of the mask support structure, can also result. Thermal shocking of the faceplate occurs, e.g., during screen fabrication when the screen surface is washed with a caustic solution at elevated temperature (60 degrees C., for example), or when it is washed with cool water when in a warm state.
Third, such thermal shocking has also been found to produce microfissures in the body of the frit filling the trough. These microfissures are believed to be caused by the extreme thermal mismatch between the metal trough and the contained frit material. Such microfissures in the frit material result in contamination of the vacuum environment within the CRT envelope after pump-down and seal-off, due to outgassing from the microfractured frit.
Alternate prior art mask support structures are disclosed in U.S. Pat. Nos. 4,737,681 and 4,745,330, both of common ownership herewith.
The '681 patent describes a mask support structure comprising a ceramic rail, on the distal edge of which is secured a metal cap providing a substrate to which a shadow mask can be welded under tension. Various configurations of ceramic rails and caps are illustrated. The ceramic element is characterized as a "buffer strip" preferably composed of a ceramic material. Quoting, "The ceramic material according to the invention is characterized by having a thermal coefficient of expansion substantially equal to the coefficient of the glass of the faceplate. The ceramic could as well have a coefficient intermediate to the coefficients of the glass and the metal hoop [the cap] effective to absorb the stresses produced due to the differing expansion and contraction coefficients of the glass and the metal hoop." (Col. 6, lines 17-37.) By way of example, the thermal coefficient of the metal cap or hoop is given as 108.times.10.sup.-7 in./in /degree C.; the ceramic element is described as having a CTC of 105.times.10.sup.-7 in./in./degree C. and the glass is said to have a CTC of 106.times.10.sup.-7 in /in./degree C.
Thus the mask support structure, having a ceramic element whose CTC is substantially equal to or intermediate to the coefficients of expansion of the glass and the metal cap would have a net CTC which is equal to or greater than that of the glass.
The later-filed '330 patent discloses a mask support structure comprising multiple layers of ceramic material of different CTC's, on the distal edge of which (remote from the faceplate inner surface) is secured a metal cap serving as a weldable substrate for attachment of the tensioned shadow mask by laser welding. The '330 patent teaches the utilization of such a structure to buffer the CTC differential between the metal, preferably Carpenter's metal, and the supporting glass surface.
Specifically, a ceramic element interfacing with the faceplate glass has a CTC equal to or higher than that of the glass. Another ceramic element interfacing with a weldable metal cap has a CTC no greater than that of the cap. Certain disclosed embodiments have one or more additional ceramic elements with CTCs between these two.
One example disclosed in the '330 patent is a three-layer ceramic system in which the element interfacing with the faceplate has a CTC of about 103.times.10.sup.-7 in./in /degree C. The next layer (away from the faceplate) has a CTC of about 105.times.10.sup.-7 in./in./degree C. The third ceramic layer has a CTC of about 107.times.10.sup.-7 in./in./degree C. The metal cap has a CTC of about 108.times.10.sup.-7 in./in./degree C.
As described therein, the ceramic material may have a composition known as forsterite (magnesium silicate). The metal cap is Carpenter Alloy No. 27 which has a CTC of 108.times.10.sup.-7 in./in./degree C. The forsterite composition can be changed by varying the composition of the ceramic. The '330 patent states that "the ceramic shadow mask support structure must provide a CTC of 100 to 110.times.10.sup.-7 in./in/degree C. to satisfy the CTC's of both the glass and the metal." In other words, the multi-layered ceramic mask support system is designed to buffer the very different CTC's of the glass and metal cap. The net CTC of the entire mask support structure would be significantly above the CTC of the glass.
Thus the '681 and '330 patents teach mask support structures which conform to the conventional wisdom of matching as closely as possible the CTC of components and glass which are affixed together in order to prevent glass defects due to strains induced by thermal coefficient mismatches.
In such suggested prior art structure, it is disclosed that by choosing the CTC of the mask support structure to be between the CTC of the faceplate glass and the weldable metal cap, a favorable buffering effect would result. The use of a mask support structure having such an intermediate CTC, however, has the effect of placing the faceplate glass beneath the structure in compression whenever the assembly is brought down in temperature from a high processing temperature.
It has been discovered that setting up a condition of compression in the sub-structure glass may be the cause of the aforedescribed glass failures. This is because, it is believed, when the sub-structure glass surface has been placed in compression, the marginal glass surface areas adjacent the ends of the mask support structures are placed in tension at the end of a cool-down phase. It is this tension in the marginal areas of the glass adjacent the ends of the mask support structure which is believed to cause the destructive glass cracks.
In accordance with the teachings of this invention, as will be described in detail hereinafter, the marginal areas of the faceplate surface immediately adjacent the ends of the mask support structures are placed in compression to thwart the initiation of glass cracks which could result in glass failures. In order to place these marginal faceplate surface areas in compression, in accordance with a preferred form of this invention, the sub-structure faceplate surface is placed in tension. This is done by causing the mask support structures to have a CTC significantly lower than that of the faceplate glass.
However, by the use of a mask support structure having a CTC significantly lower than that of the supporting faceplate glass, there exists the possibility that spalling or other strain-related glass defects may occur at or near the interface of the faceplate glass and the mask support structure.
A second concern involves the difficulty in fine tuning the net CTC of the mask support structure to achieve the exact level of strain desired to exist in the sub-structure faceplate glass.
As will be explained below, in the preferred form of the invention, the mask support structure is composed of fosterite ceramic and its CTC is controlled by varying the content of MgO in the composition.
However a small change in the MgO content produces a relatively large change in CTC. In a mass production environment it is difficult to consistently obtain a ceramic mask support structure having the exact CTC desired by varying the MgO content of a fosterite composition.
In accordance with an aspect of this invention, both of these concerns having to do with the CTC of the ceramic mask support structure (i.e., mismatch to glass and CTC control) are answered--first by the use of a cement having a CTC intermediate that of the ceramic element and the faceplate, and the second, by the provision of a controlled cross-section recess or groove in the base of the ceramic element.
The cement, provided in a controlled amount (by virtue of the recess), 1) buffers the CTC mismatch between the glass and the mask support structure, and 2) fine-tunes the net CTC of the mask support structure.